1. Field of the Invention
The present invention relates to a method of fabricating a semiconductor wafer obtained by polishing the surface thereof after alkali etching.
2. Description of the Prior Art
Working strains are produced on the surface and reverse surface of a semiconductor wafer planarized by means of lapping or surface grinding, and thus in order to remove the working strains, conventionally the semiconductor wafer is subjected to etching. The etching solution of the etching can be an alkali etching solution of KOH or NaOH or an acid etching solution of mixed acids.
A comparasion of each etching is described herinafter. For example, in the case of using a 40% aqueous solution of NaOH as an alkali etching soltuion, the etching rate is 4.0 .mu.m/min. On the other hand, in the case of using a 40% mixed acid as an acid etching solution, the etching rate is 10 .mu.m/min. Accordingly, even for etching solutions of the same concentration, the variation in rate of acid etching is quite large.
Furthermore, as a feature of alkali etching, as stated above, not only is the etching rate low for alkali etching in comparison with acid etching, the surface P-V value (P-V: Peak to Valley) is high, as can be seen from FIG. 4. That is, roughness is high, and thus the planarity obtained by lapping can be maintained. P-V value represents a deference between peak height and valley height.
On the other hand, the etching rate is high when an acid etching is used, and thus the planarity is easily damaged and the TTV (TTV: Total Thickness Variation) value is high. On the contrary, acid etching has the features that the surface is smooth and the P-V value is small. As a means for ensuring the planarity by the acid etching, a spin etching method is used. The method involves etching semiconductor wafers one piece at a time, and thus has poor throughput when compared with a batchwise method.
Accordingly, in attempt to increase the yield of the semiconductor device process, in the batchwise etching process used in recent years, alkali etching, which is readily able to maintain the planarity, is widely used.
However, the rate of removing metal contaminants is low for alkali etching, and thus if there are metal contaminants on the surface of semiconductor wafers before alkali etching, such metal contaminants remain on the wafers even after they are subjected to alkali etching.
Moreover, aqueous solutions of metal hydroxides such as KOH or NaOH are widely used as alkali etching solutions, and as a result, metal ions such as potassium and sodium ions remain on the wafers during etching, and become metal contaminants.